The present invention relates to data communications, and in particular to a method for transmitting data on a bus connecting a main unit of an electrical controller to at least one secondary unit.
Data transmission methods of the type to which the present invention is directed are commonly used for modular programmable controllers. Depending on the controller, a suitable bus may be designed as either a serial bus or a parallel bus. An example of a programmable controller with a serial bus is the "SIEMENS" programmable controller "SIMATIC S5-100 U"; and an example of a programmable controller with a parallel bus is the programmable controller "SIMATIC S5-115 U".
Industrial systems and processes are often controlled by programmable controllers. Transmission errors occurring between the main unit and a secondary unit of such a controller can lead to faulty system or process control, which can cause damage to material and/or injury to persons. Therefore, reliable data transmission between the main unit and its secondary units is absolutely essential, even under the harsh conditions of an industrial operation.
Known approaches to data transmission in industrial systems have used increasingly more complex bus interface connections to guarantee reliable data transmission, leading to highly complex hardware and increased costs. Even with this high level of complexity, however, the transmitted information is not validated. Instead, prior art methods merely check whether a secondary unit responds within a required reaction time after having been addressed by the main unit. If a response is not timely received, an error message is generated prompting execution of a user-programmed error recovery routine. Depending on the particular error recovery routine, the programmable controller may be stopped, or the failure of the addressed secondary unit to respond may be ignored.